184 



STRATIFIED OR SEDIMENTARY ROCKS. 



(b.) Association with Foldings. — Cleavage is always associated with 

 strong foldings and contortions of the strata. The folding of the 

 strata is produced by horizontal pressure ; the 

 strike of the strata, or the direction of the anti- 

 clinal and synclinal axes, being of course at right 



B 



Fig. 161.— Cardium Hillanum: A, natural form; B and C, deformed by pressure. 



angles to the direction of pressure. ISTow, if cleavage is produced by 

 the same pressure which folded the strata, then in this case we ought 



to find the cleavage-planes 

 highly inclined, and their 

 strike parallel with the 

 strike of the strata ; and 

 such we find is usually the 

 fact. In Fig. 162 the 

 heavy lines represent the 

 strata and the light lines 

 a nearly level surface, and 



Fig. 162.— Cleavage-Planes intersecting Strata. 



the cleavage-planes, both outcropping on 



parallel to each other. 



(c.) Association with Contorted Laminw. — The last evidence was 

 taken from foldings on a grand scale of the crust of the earth ; but 

 even fine lines of lamination are often thrown into 

 intricate foldings by squeezing together in the di- 

 rection of the lamination-planes. In such case, of 

 course, the cleavage ought by theory to be at right 

 angles to the original direction of the lamination, 

 and in such direction we actually find them. Fig. 

 163 represents a block of rock in which three 

 lamination-lines are visible. The lower one, / d, 

 consists of coarse sand which could not mash, and 

 therefore has been thrown into folds. As the 

 specimen stands in the figure, the pressure has 

 been horizontal ; the perpendicular lines represent 

 the position of the cleavage-planes. Fig. 164 rep- 

 resents a beautiful specimen of laminated slate, in 

 which the lamination-planes have been thrown into folds by pressure. 

 The direction of the pressure is obvious. The planes of cleavage are 

 parallel to the face, cp, and therefore at right angles to the pressure. 



(d.) Flattened Nodules. — In some finely-cleaved slates, such as are 

 used for writing-slates, it is common to find small light-greenish, ellip- 



a 



iPfii 



111 



I 



''fill 



i'lililll; 111 ", 



■1 



C 



Mm 



■ 



sS II 



a 



\jyjk/J 





iwjA Rjf 





hi 



/m\ 



|/k|]y/ 



e 



111 



11 



Hi 



Fig. 



163.— Cleavage-Planes 

 (after Tyndall). 



